Self-Aligned BSI Sensor Grids for Optical Isolation
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Solution Overview
Problem
The fabrication of back side illumination (BSI) image sensors faces challenges such as silicon damage from dry etching processes and misalignment of metal grids with trench isolation grids, which affect optical isolation and performance.
Innovation Solution
A method is developed to self-align the metal grid with the trench isolation grid without using dry etching, ensuring direct contact and reducing silicon damage, thereby enhancing optical performance and reducing white pixel counts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional fabrication processes are used to create the metal grid, then the manufacturing process can be completed, but silicon damage occurs and misalignment between the metal grid and trench isolation grid reduces optical isolation performance
Solution Approach 1:
The patent applies preliminary action by forming the metal grid pattern in the same lithography step that defines the trench isolation grid pattern. The metal grid is deposited and patterned before the trenches are etched, ensuring that the metal grid positions are pre-determined and will self-align with the subsequent trench formation. This preliminary patterning eliminates the need for separate alignment steps and prevents silicon damage from multiple lithography cycles.
Solution Approach 2:
The fabrication process utilizes self-service through self-aligned fabrication where the metal grid automatically aligns with the trench isolation grid without requiring additional alignment operations. The process is designed so that the metal grid pattern serves as its own alignment reference, and the trench etching process naturally follows the metal grid pattern, creating a self-correcting system that eliminates misalignment errors.
2Reliability
If multiple separate fabrication steps are used to create the metal grid and trench isolation grid, then flexibility in process optimization is maintained, but alignment errors accumulate and optical isolation performance deteriorates
Solution Approach 1:
The patent merges the metal grid formation and trench isolation grid formation into a single integrated fabrication sequence. The lithography step that patterns the metal grid simultaneously defines the trench locations, and the metal grid structure serves as the alignment template for subsequent trench etching. This merging of operations ensures perfect alignment while actually reducing the number of independent process modules required.
Solution Approach 2:
The metal grid pattern acts as an intermediary element that mediates between the lithography process and the trench etching process. By using the metal grid as an intermediate alignment reference, the patent creates a bridge that ensures precise positioning of the trenches relative to the metal grid without requiring complex direct alignment mechanisms.
Data Source
AI summary
A method of fabricating self-aligned grids in a BSI image sensor is provided. The method includes depositing a first dielectric layer over a back surface of a substrate that has a plurality of photodiodes formed therein, forming a grid of trenches, and filling in the trenches with dielectric material to create a trench isolation grid. Here, a trench passes through the first dielectric layer and extends into the substrate. The method further includes etching back dielectric material in the trenches to a level that is below an upper surface of the first dielectric layer to form recesses overlaying the trench isolation grid, and filling in the recesses with metallic material to create a metallic grid that is aligned with the trench isolation grid.